1. Field of the Invention
This invention relates to a process for producing a porous ceramic and porous ceramic composite structure having interconnected porosity, in particular a ceramic filter, which utilizes combustion synthesis, also known as self-propagating high temperature synthesis, thereby obviating a high temperature firing step required in the conventional process.
Ceramic filters are used for filtering undesirable particles from a molten metal before the metal is poured into a pattern or mold. Porous ceramic articles are also in use, e.g., as substrates for catalysts, removal of contaminants from exhaust gases, collecting impurities from highly corrosive acids and chemicals, in kiln furniture, or aerospace applications. Ceramic filters are also used as structural materials in low density load bearing structures.
2. Description of the Prior Art
U.S. Pat. No. 3,090,094, issued May 21, 1963 to K. Schwartzwalder et al, discloses a method of making an open-cell porous ceramic article which comprises immersing an open-cell spongy material, preferably polyurethane, in a slurry containing a ceramic coating material to coat cell-defining walls of the spongy material, removing excess slurry from the spongy material, and firing the coated spongy material at a temperature and for a time sufficient to remove the spongy material and form a hardened, vitrified structure. The ceramic coating material may include particulate zirconia, zircon, petalite, mullite, talc, silica and alumina, having particle sizes ranging from -80 mesh to -600 mesh. A binder such as clay, sodium silicate, and calcium aluminate and phosphoric acid, is preferably present in the slurry. Firing is conducted at 500.degree. to 3000.degree. F. (260.degree. to 1650.degree. C.), preferably at 2100.degree. to 2950.degree. F. (1150.degree. to 1620.degree. C.).
U.S. Pat. No. 3,097,930, issued Jul. 16, 1963 to I. J. Holland, discloses a method of making a porous shape of sintered refractory material which comprises impregnating a foamed plastic sponge shape with a suspension of refractory particles, drying the impregnated shape, and firing the dried shape in an inert atmosphere to volatilize the sponge material and to sinter the refractory particles. The impregnation and drying steps may be repeated. The foamed plastic sponge may be polystyrene, polyethylene, polyvinyl chloride, latex, or polyurethane, the latter being preferred. Refractory materials include clays, minerals, oxides, borides, carbides, silicides, nitrides and mixtures thereof. Specific examples used alumina, beryllia and china clay with particle sizes ranging from less than 1 to greater than 10 microns. Firing was conducted at 1700.degree. C. for alumina and 1350.degree. C. for china clay.
U.S. Pat. No. 4,697,632, issued Oct. 6, 1987 to N. G. Lirones, discloses a ceramic foam filter, insulating refractory lining, and a melting crucible, and a process for production thereof, which comprises providing an open-cell foam pattern, impregnating the pattern with a ceramic slurry, burning out the foam pattern at a temperature between 1400.degree. and 2200.degree. F. (760.degree. and 1205.degree. C.) to form a ceramic substrate, impregnating the ceramic substrate with additional ceramic slurry, and firing the impregnated ceramic substrate at a temperature of 2200.degree. to 3400.degree. F. (1205.degree. to 1870.degree. C.). The foam pattern material may be a flexible polyurethane, polyethylene, polypropylene or graphite. A suitable ceramic slurry contains from 1% to 20% silica (dry weight), and from 99% to 80% alumina (dry weight), with a viscosity between 5 and 20 seconds and a film weight between 1.0 and 8.0 grams per standard six inch square plate. Preferably the slurry includes a suspending agent, a wetting agent and a defoaming agent. Zirconia may also be used as ceramic material.
U.S. Pat. No. 3,111,396, issued Nov. 19, 1963 to B. B. Ball, discloses a method of making a porous metallic article which comprises impregnating a porous organic structure with a suspension of powdered metal, metal alloy or metal compound, and binder, slowly drying the impregnated structure, heating at about 300.degree.-500.degree. F. (150.degree.-260.degree. C.) to char the organic structure, and then heating at about 1900.degree. to about 3000.degree. F. (1040.degree. to 1650.degree. C.) to sinter the powder into a porous material.
Other United States patents relating to porous ceramic filters and methods for making them include:
______________________________________ 3,893,917 - July 8, 1975 - M. J. Pryor et al 3,947,363 - March 30, 1976 - M. J. Pryor et al 3,962,081 - June 8, 1976 - J. C. Yarwood et al 4,024,056 - May 17, 1977 - J. C. Yarwood et al 4,081,371 - March 28, 1978 - J. C. Yarwood et al 4,257,810 - March 24, 1981 - T. Narumiya 4,258,099 - March 24, 1981 - T. Narumiya 4,391,918 - July 5, 1983 - J. W. Brockmeyer ______________________________________
All these patents require firing the dried filter at high temperature in order to sinter the ceramic material.
"Simultaneous Preparation and Self-Sintering of Materials in the System Ti-B-C", J. W. McCauley et al, Ceramic Eng. & Sci. Proceedings, 3, 538-554 (1982), describes self-propagating high temperature synthesis (SHS) techniques using pressed powder mixtures of titanium and boron; titanium, boron and titanium boride (TiB.sub.2); and titanium and B.sub.4 C. Stoichiometric mixtures of titanium and boron were reported to react almost explosively (when initiated by a sparking apparatus) to produce porous, exfoliated structures. Reaction temperatures were higher than 2200.degree. C. Mixtures of titanium, boron and titanium boride reacted in a much more controlled manner, with the products also being very porous. Reactions of titanium with B.sub.4 C produced material with much less porosity. Particle size distribution of the titanium powder was found to have an important effect on the process, as was the composition of the mixtures. Titanium particle sizes ranging from about 1 to about 200 microns were used.
"Effects of Self-Propagating Synthesis Reactant Compact Character on Ignition, Propagation and Resultant Microstructure", R. W. Rice et al, Ceramic Eng. & Sci. Proceedings, 7, 737-749 (1986), describes SHS studies of reactions using titanium powders to produce TiC, TiB.sub.2, or TiC+TiB.sub.2. Reactant powder compact density was found to be a major factor in the rate of reaction propagation, with the maximum rate being at about 60.+-.10% theoretical density. Reactant particle size and shape were also reported to affect results, with titanium particles of 200 microns, titanium flakes, foil or wire either failing to ignite or exhibiting slower propagation rates. Particle size distribution of powdered materials (Al, B, C, Ti) ranged from 1 to 220 microns. Tests were attempted with composites of continuous graphite tows infiltrated with a titanium slurry, but delamination occurred. Tests with one or a few tows infiltrated with a titanium powder slurry (to form TiC plus excess Ti) were able to indicate a decrease in ignition propagation rates as the thermal conductivity of the environment around the reactants increases, leading to a failure to ignite when local heat losses are too high.
H. C. Yi et al, in Jour. Materials Science, 25, 1159-1168 (1990), review SHS of powder compacts and conclude that almost all the known ceramic materials can be produced by the SHS method for applications such as abrasives, cutting tools, polishing powders; elements for resistance heating furnaces; high temperature lubricants; neutron alternators; shape-memory alloys; steel melting additives; and electrodes for electrolysis of corrosive media. The need for considerable further research is acknowledged, and major disadvantages are pointed out.
This article further reports numerous materials produced by SHS and combustion temperatures for some of them, viz., borides, carbides, carbonitrides, nitrides, silicides, hydrides, intermetallics, chalcogenides, cemented carbides, and composites.
Combustion wave propagation rate and combustion temperature are stated to be dependent on stoichiometry of the reactants, pre-heating temperature, particle size and amount of diluent.
U.S. Pat. No. 4,459,363, issued Jul. 10, 1984 to J. B. Holt, discloses synthesis of refractory metal nitride particles by combustion synthesis of an alkali metal or alkaline earth metal azide with magnesium or calcium and an oxide of Group III-A, IV-A, III-B, or IV-B metals (e.g., Ti, Zr, Hf, B and Si), preferably in a nitrogen atmosphere.
U.S. Pat. No. 4,909,842, issued Mar. 20, 1990 to S. D. Dunmead et al, discloses the production of dense, finely grained composite materials comprising ceramic and metallic phases by self-propagating high temperature synthesis (SHS) combined with mechanical pressure applied during or immediately after the SHS reaction. The ceramic phase or phases may be carbides or borides of titanium, zirconium, hafnium, tantalum or niobium, silicon carbide, or boron carbide. Intermetallic phases may be aluminides of nickel, titanium or copper, titanium nickelides, titanium ferrides, or cobalt titanides. Metallic phases may include aluminum, copper, nickel, iron or cobalt. The final product has a density of at least about 95% of the theoretical density and comprises generally spherical ceramic grains not greater than about 5 microns in diameter in an intermetallic and/or metallic matrix. Interconnected porosity is not obtained in this product, nor does the process control porosity.
The well known thermit reaction involves igniting a mixture of powdered aluminum and ferric oxide in approximately stoichiometric proportions which reacts exothermically to produce molten iron and aluminum oxide.
The conventional process for producing porous ceramic filters and structural parts requires long soaking periods at very high temperatures. There are two serious disadvantages in this process: first, the energy consumption is high; secondly, soaking the ceramic foam (particularly larger structures) at very high temperature results in sagging and warping of the structure, thus losing dimensional tolerance. Moreover, the ceramic composition resulting from a conventional mixture of silica and alumina may not be capable of withstanding very high thermal shock, unless a cordierite phase (composed of silica, alumina and magnesia) is present.
There is therefore a definite need for an improved process for the production of porous ceramic structures, despite the numerous patents acknowledged above.
To the best of applicants' knowledge, there has been no suggestion in the prior art of the possibility of utilizing combustion synthesis techniques in the fabrication of porous ceramic structures involving the impregnation of a foamed polymer with ceramic precursors to obtain interconnected porosity.